Magnesium deprivation affects development and biomineralization in the sea urchin arba-cia lixula

Skeletogenesis is a key morphogenetic event in the life of marine invertebrates. Marine calcifiers secrete their calcareous skeletons taking up ions from seawater. Marine biominerals include aragonite and calcite, the latter of which in some taxa (e.g. echinoderms, coralline algae) can have a substantial magnesium (Mg) component. Echinoderms have an extensive endoskeleton composed of high magnesian calcite and occluded matrix proteins1. As biomineralization in sea urchin larvae is sensitive to the Magnesium:Calcium ratio of sea water, we investigated the effects of magnesium deprivation on development and skeletogenesis in the Mediterranean sea urchin Arbacia lixula. Microscopic inspection revealed that embryos reared in Mg-free seawater exhibited developmental delay from 6 hours post-fertilization, complete lack of skeleton formation at 24 hours, and severe skeleton malformations in larvae (48-72 hours). We subsequently focused on the localization of the skeletogenic cells (primary mesenchyme cells) and the spatial expression of associated genes. Immunocytochemistry revealed abnormal ectopic location of the primary mesenchyme cells (PMCs) and of the developing skeleton of treated embryos. Expression of msp130, an important skeleton matrix protein gene expressed only in PMCs, detected by in situ hybridization, was normal at 24 hours, but this gene was not down-regulated at 48 hours, as in controls2. Strikingly, development of the pigment cells, immune cells that, like the skeleton, are mesodermal derivatives, was also impaired. These results suggest the essential role of Mg in skeleton formation in sea urchin embryos with an indication that this element is also generally important for development of mesoderm. 1. Smith AM et al. Mar Ecol Prog Ser 2016, 561:1–16 2. Martino C et al. Aq tox 2018, 194:57-66.